1. Field of the Invention
This invention relates to an information signal record medium of the capacitance type. More specifically, this invention relates to an information signal record medium of the capacitance type having good stability properties wherein an information signal is recorded as a variation in the geometrical shape of its surface area, and is played back by utilizing a variation in the capacitance between its surface and an electrode of a scanning reproducing stylus, characterized in that at least the surface area of the record medium is formed of a material composed of an electrically conductive substance and a specified resin having good compatibility with it.
2. Description of the Prior Art
A so-called capacitance reproducing method has previously been known which comprises causing a reproducing stylus having an electrode to scan relatively a recording track of an information signal record medium having an information signal recorded on its flat surface or in a groove thereof as a variation in geometrical shape, thereby allowing the capacitance between the electrode of the reproducing stylus and the record medium to vary according to the variation in geometrical shape.
A conventional information signal record medium of the capacitance type used in this type of capacitance reproducing method is produced by adhering a thin metal film having a thickness of several hundred A as an electrode for providing a capacitance between the electrode and an electrode of a reproducing stylus to the surface of a record medium substrate press-molded in a varying geometrical shape, and further adhering thereto a thin dielectric film having a thickness of several hundred A in order to protect the thin metal film, to prevent short-circuiting between the electrodes, and to increase a dielectric constant between the electrodes. The production of the conventional information signal record medium of the capacitance type requires many process steps including a step of press-molding the substrate, a step of adhering the thin metallic film and a step of adhering the dielectric film. Hence, the manufacturing process is complex and requires large-sized manufacturing facilities. This necessarily leads to a very high cost of production.
An improved information signal record medium of the capacitance type which eliminates these defects was proposed in Japanese Laid-Open Patent Publication No. 119017/78. This record medium is produced by press-forming an electrically conductive plastic material composed of a vinyl chloride/vinyl acetate copolymer and a minor amount of carbon black as a conductive substance, whereby an information signal is recorded as a variation in geometrical shape. Since a capacitance is generated between the record medium and an electrode of a reproducing stylus, the step of adhering a thin metallic film essential in the prior art can be obviated. Furthermore, since the fine particles of carbon black themselves are coated with the resin, the step of adhering a dielectric film essential in the prior art can be omitted. Hence, the information signal record medium of the capacitance type can be produced very easily at a very low cost. The production of this record medium, however, has the disadvantage that because a friction which occurs between the carbon black particles and the resin molecules during the mixing of the vinyl chloride/vinyl acetate copolymer with carbon black produces a high internal heat, the copolymer is liable to undergo thermal decomposition during the processing. In addition, since the melt viscosity of the material is high because of the poor affinity between the carbon particles and the resin molecules, the material cannot be well molded by conventional molding facilities.
In order to overcome such disadvantages, it may be possible to increase the amount of a lubricant or plasticizer component to be incorporated into the resin. In such a case, however, new problems arise. For example, the record medium may be deformed when the atmospheric temperature rises even a little (namely, the environmental stability of the record medium is poor), and the performance of the record medium is reduced owing to the bleeding of the plasticizer, etc. (namely, the stability with time of the record medium is poor).